1. Field of the Invention
The present invention relates to an optical head of an optical recording/reproducing apparatus, particularly to a thin optical head having preferable optical characteristics.
2. Description of the Related Art
An optical head is used as an important component for reading a signal from an optical recording medium such as an optical disk or optical card memory, that is, a compact disk (CD) or DVD. It is preferable that an optical head is provided with not only a signal detecting function but also a control mechanism such as a focus servo or tracking servo in order to fetch a signal from an optical recording medium.
FIG. 19 shows a conventional typical optical head. As shown in FIG. 19, a laser beam 2 emitted from a semiconductor laser 1 serving as a light source is changed to parallel rays by a collimator lens 3, passes through a focus/track error signal detecting optical element 8 constituted of a holographic optical element, then the optical axis of the light is bent up to 90xc2x0 and the light enters an objective lens 4. The laser beam 2 condensed on an optical disk 11 by the objective lens 4 is reflected and goes back through the original optical path, changed to parallel rays by the objective lens 4, and reflected by a mirror 20 to enter the focus/track error signal detecting optical element 8. The laser beam 2 entering the focus/track error signal detecting optical element 8 is divided in the element 8 and condensed on a photodetector. Thereby, a reproduced signal and focus and track error signals serving as servo signals are read.
As shown in FIG. 19, the height of the optical head is shown by the sum of a WD (working distance), thickness of the objective lens 4, space from the bottom face of the objective lens 4 up to the top of the mirror 20, and height of the mirror 20.
To decrease the height of an optical head, the minimum value of the sum of the WD, lens thickness, and space is almost determined by the type of the optical disk 11. In the case of a DVD, even if respectively setting the WD, lens thickness, and space to a minimum value of 1.1 mm, it is necessary to set the height of the mirror 20 to a value larger than a beam diameter, that is, it is necessary to set the height to at least 3 mm. Therefore, in this case, the height of the optical head is estimated as at least 6.3 mm and thus, it is difficult to further decrease the height of an optical head.
The present invention is made to solve the above problems of the prior art and its object is to provide an optical head which can be decreased in thickness and which has preferable optical characteristics.
One aspect of the present invention is an optical head comprising:
a diffractive optical element set in the optical path between a light source and an information recording medium; and
refractive optical means which is set in the optical path and into which the optical axis of the light emitted from the light source diagonally comes; wherein
diffraction angle change of the diffracted light emitted from the diffractive optical element due to the wavelength fluctuation of the emitted light and refraction angle change of the refracted light emitted from the refractive optical means occur in the direction in which the diffraction angle change and refraction angle change are canceled.
For example, when using a semiconductor laser beammaking it possible to constitute a thin optical head and serving as the light emitted from a light source, it is possible to obtain a preferable condensed spot on an optical disk surface even if central wavelengths of the emitted light are changed due to spread of a wavelength band of approx. 2 nm due to a high-frequency superimposition module or change of environmental temperatures.
Another aspect of the present invention is an optical head, wherein the diffractive optical element is integrated with the refractive optical means.
Thereby, for example, the structure is stabilized and alignment is easily made.
Still another aspect of the present invention is an optical head, wherein collimator means is included which changes the light emitted from the light source substantially to parallel rays and makes the parallel rays enter the diffractive optical element.
Thereby, for example, the diffraction efficiency and diffraction-angle change value of the light coming into a diffractive optical element become equal over the entire surface.
Yet another aspect of the present invention is an optical head, wherein a focus/track error signal detecting optical element is set between the light source and the refractive optical means and the diffractive optical element is integrated with the focus/track error signal detecting optical element.
Thereby, for example, the structure is stabilized and alignment is easily made.
Still yet another aspect of the present invention is an optical head, wherein the diffractive optical element is a uniform-period grating.
Thereby, for example, a diffractive optical element is easily aligned or fabricated.
A further aspect of the present invention is an optical head, wherein
the diffractive optical element is set in a convergence-light optical path or divergence-light optical path, and
the period of the diffractive optical element differs depending on places in accordance with the convergent degree or divergent degree of the light coming into the diffractive optical element.
xe2x80x9cA still further aspectxe2x80x9d of the present invention is an optical head, wherein the period is adjusted so as to be further lengthened substantially from the central portion toward the circumferential portion of the diffractive optical element.
Thereby, for example, it is possible to accurately uniform the diffraction angle change value of the diffracted light emitted from a diffractive optical element over the entire surface.
A yet further aspect of the present invention is_an optical head, wherein the diffractive optical element is set in a convergent-light optical path or divergent-light optical path having a numerical aperture of 0.39 or less and the period of the diffractive optical element is uniform.
Thereby, for example, a diffractive optical element is easily aligned or fabricate.
A still further aspect of the present invention is an optical head, wherein the diffractive optical element is set in an optical path nearby the light source.
Thereby, for example, it is possible to decrease the area and the cost of a diffractive optical element.
One aspect of the present invention is an optical head, wherein the refractive optical means is an optical element having three planes for receiving or reflecting light and the diffractive optical element is formed on at least one of the three planes of the refractive optical means.
Thereby, for example, the structure is stabilized and alignment is easily made.
Another aspect of the present invention is an optical head, wherein the diffractive optical element is the reflective type and is formed on the reflective plane of the refractive optical means.
Thereby, for example, the diffraction efficiency of a diffractive optical element is improved.
Still another aspect of the present invention is an optical head, wherein the refractive optical means is a prism made of low-dispersion transparent material having three planes for receiving or reflecting light.
Yet another aspect of the present invention is an optical head, wherein the transparent material has an Abbe number of 50 or more.
Thereby, for example, because the period of a diffractive optical element is lengthened, the element is easily fabricated, a high diffraction efficiency is obtained, and it is possible to cancel influences of wavelength fluctuation in a wide wavelength region.
Still yet another aspect of the present invention is an optical head, wherein
the refractive optical means is a prism made of transparent material having a refractive index n, and
one of the base angles of the prism is substantially a right angle and the other xcex8 of the base angles has an angle xcex8 substantially meeting xcex8=nxc2x7sin(3xcex8xe2x88x9290xc2x0).
Thereby, for example, it is possible to make the optical axis of the light coming into refractive optical means almost perpendicular to that of the light emitted from the refractive optical means.
A further aspect of the present invention is an optical head, wherein
the refractive optical means is a prism made of transparent material having a refractive index n, and
one xcex8 of the base angles of the prism substantially meets sin(2xcex8xe2x88x9245xc2x0)=1/nxc2x7sin xcex8 and the other xcex81 of the base angles meets xcex8+85xc2x0xe2x89xa6xcex81xe2x89xa6xcex8+95xc2x0.
Thereby, for example, it is possible to make the beam diameter of the light coming into refractive optical means almost equal to that of the light emitted from the refractive optical means and make optical axes of the former and latter lights almost perpendicular to each other.
A still further aspect of the present invention is an optical head, wherein the light source has a plurality of light-source sections for emitting wavelengths different from each other.
Thereby, for example, it is possible to correspond to a plurality of types of information recording media.
A yet further aspect of the present invention is an optical head, wherein the diffractive optical element is set only to an optical path nearby a light-source section for emitting minimum-wavelength light among the light-source sections.
Thereby, for example, it is possible to decrease the cost and optimize the optical characteristic of a short wavelength most subject to wavelength fluctuation.
A still yet further aspect of the present invention is an optical head, wherein
the diffractive optical element has a blazed sectional form, and
when assuming the minimum value of the different wavelengths as xcex1 and the maximum value of them as xcex2 and the refractive index of the diffractive optical element as n, the groove depth L of the diffractive optical element meets the relation of xcex1/(nxe2x88x921)xe2x89xa6Lxe2x89xa6xcex2/(nxe2x88x921).
Thereby, for example, it is possible to raise the diffraction efficiency of a diffractive optical element for a plurality of wavelengths.
One aspect of the present invention is an optical head, wherein the groove depth L of the diffractive optical element is substantially equal to (xcex1+xcex2)/[2(nxe2x88x921)].
Thereby, for example, it is possible to raise the diffraction efficiency of a diffractive optical element for a plurality of wavelengths at the best balance.
Another aspect of the present invention is an optical head, wherein
the sectional form of the diffractive optical element is a multilevel shape with p levels, and
when assuming the minimum value of the different wavelengths as xcex1 and the maximum value of them as xcex2 and the refractive index of the diffractive optical element as n, the groove depth L of the diffractive optical element meets the relation of (pxe2x88x921)xc2x7xcex1/[pxc2x7(nxe2x88x921)]xe2x89xa6Lxe2x89xa6(pxe2x88x921)xc2x7xcex2/[pxc2x7(nxe2x88x921)].
Thereby, for example, it is possible to raise the diffraction efficiency of a diffractive optical element for a plurality of wavelengths.
Still another aspect of the present invention is an optical head, wherein the groove depth of the refractive optical element is substantially equal to (pxe2x88x921)xc2x7(xcex1+xcex2)/[2p(nxe2x88x921)].
Thereby, for example, it is possible to raise the diffraction efficiency of a diffractive optical element for a plurality of wavelengths at the best balance.
Yet another aspect of the present invention is an optical head, wherein
the refractive optical element is a prism made of transparent material having a refractive index n, and
when assuming a setting angle formed between the bottom face and the setting reference plane of the refractive optical means as xcex8b and an angle formed between the light coming into the refractive optical means from the light source and the setting reference plane as xcex8p, one angle xcex8 of the base angles of the prism substantially meets sin(xcex8xe2x88x92xcex8b)=nxc2x7sin(4xcex8xe2x88x922xcex8bxe2x88x92xcex8pxe2x88x9290xc2x0xe2x88x92xcex8xe2x80x2) and nxc2x7sin xcex8xe2x80x2=sin(xcex8xe2x88x92xcex8b) and the other angle xcex81 of the base angles substantially meets xcex81=xcex8+90xc2x0xe2x88x922xcex8bxe2x88x92xcex8p.
Still yet another aspect of the present invention is an optical head, wherein the xcex8b substantially meets 2xc2x0xe2x89xa6xcex8bxe2x89xa610xc2x0.
Thereby, for example, it is possible to further decrease the height of an optical head.
A further aspect of the present invention is an optical head, wherein
the refractive optical means is a prism having three optical panes, and
when assuming one of the three planes at the information recording medium side as a first plane, one of them at the light-source side as a second plane, and remaining one of them as a third plane, the light emitted from the light source passes through the second plane, reflects on the first and
Thereby, for example, it is possible to decrease the height of an optical head in accordance with zigzag propagation of an optical path in a prism.
A still further aspect of the present invention is an optical head comprising refractive optical means provided with a prism having three optical planes in the optical path between a light source and information recording medium; wherein
when assuming one of the three planes at the information recording medium side as a first plane, one of them at the light-source side as a second plane, and the remaining one of them as a third plane, the light emitted from the light source passes through the second plane, reflects on the first and third planes in order, and passes through the first plane.
Thereby, for example, it is possible to decrease an optical head in thickness.
A yet further aspect of the present invention is an optical head, wherein the wavelength xcex of the emitted light substantially meets 0.35 xcexcmxe2x89xa6xcexxe2x89xa60.5 xcexcm.
A still yet further aspect of the present invention is an optical head, wherein the third plane is parallel with the setting reference plane of the refractive optical means.
One aspect of the present invention is an optical head, wherein
an objective lens is set in the optical path between the information recording medium and the refractive optical means, and
the height of the uppermost portion of the emitted light passing through the second plane from the setting reference plane of the refractive optical means is higher than the height of the lowermost portion of the objective lens from the setting reference plane.